Recently, a technique of quantizing a phase of light by utilizing an optical parametric process which is one of nonlinear optical phenomena has been developed (J. Kakande, R. Slavik, F. Parmigiani, A. Bogris, D. Syvridis, L. Gruner-Nielsen, R. Phelan, P. Petropoulos, and D. J. Richardson, “Multilevel quantization of optical phase in a novel coherent parametric mixer architecture,” Nature Photon. 5, 748-752 (2011)).
The quantization of the optical phase is an important technique that can be utilized in large-capacity optical communication, a high-speed A-D converter, or the like which supports today's advanced information society.
In particular, it is possible to perform an operation such as phase regeneration of multilevel phase shift keying signal light and quadrature phase component separation in the optical communication.
FIG. 1 illustrates a concept that quantizes an optical phase to M levels.
FIG. 1(a) illustrates a relation between a phase of input light and a phase of output light, and FIG. 1(b) illustrates a relation between a phase of input light and a power of output light.
Input and output characteristics illustrated in FIG. 1(a) are to have an effect of suppressing phase noise with respect to M-level phase shift keying (M-PSK) signal light.
In particular, characteristics in the case of M=2 are called quadrature phase squeezing and can be utilized not only for phase regeneration of a BPSK signal but also for separation of a quadrature phase component of the multilevel phase shift keying (n-QAM) signal light.
The quantization of the optical phase can be performed through use of a phase sensitive optical amplifier (Phase Sensitive Amplifier: PSA).
There are various types of the PSA, and the most representative one is a system that is called a dual pump PSA (DP-PSA) in which signal light is multiplexed with two pump light beams, and the multiplexed signal light is introduced into a nonlinear optical medium (WO2013111413).
FIG. 2 illustrates wavelength arrangement at the time of performing the phase regeneration of the BPSK signal through use of the DP-PSA.
Two pump light beams are arranged such that an average frequency thereof corresponds to a frequency of the signal light.
When the phase of the signal light is 0 or π relative to the average phase of the two pump light beams, a positive optical parametric gain is generated in the nonlinear medium, and the signal light is amplified.
In contrast, when the phase of the signal light is π/2 and −π/2, a negative optical parametric gain is generated, and the signal light is attenuated.
That is, the quadrature phase squeezing occurs in which an in-phase component of the signal light is amplified and a quadrature phase component is attenuated.
FIG. 3 is a constellation diagram of the BPSK signal where a horizontal axis indicates an in-phase axis and a vertical axis indicates a quadrature phase axis, and FIG. 3 illustrates a state in which the phase noise is suppressed by the quadrature phase squeezing.
It is also possible to perform phase regeneration of a QPSK signal through use of the DP-PSA in the same manner as that of the BPSK signal, and wavelength arrangement in this regard is illustrated in FIG. 4.
Two pump light beams are arranged such that a frequency difference between pump light 2 and signal light is three times a frequency difference between pump light 1 and the signal light.
When phase differences between the signal light and the two pump light beams are 0, ±π/2, and π, a positive optical parametric gain is generated, and the signal light is amplified.
In contrast, when the phase difference is ±π/4 and ±3π/4, a negative optical parametric gain is generated, and the signal light is attenuated.
In addition, the PSA is operated in a saturation region, so that simultaneous regeneration of a phase and an amplitude of a phase shift keying signal can also be performed.
An important indicator of the quantization performance is an amount of gain variation accompanying a phase change, that is, a so-called gain extinction ratio (GER).
In general, a high GER of equal to or higher than 25 dB is required in order to perform the quadrature phase squeezing.
It is difficult to achieve such a high GER in the PSA unless a high optical parametric gain is obtained.
Accordingly, a nonlinear optical element with a great nonlinearity, and further, a high breakdown threshold capable of withstanding introduction of high power pump light is required.
Highly nonlinear optical elements having such characteristics have been particularly developed and used in successful examples that have been made so far.